1. Field of the Invention
The invention relates generally to printing, and more particularly to a method and apparatus for effecting temperature-controlled airflow to accelerate setting and drying of inks, varnishes, or coatings on stock in a printing operation.
2. Description of Related Art
In printing operations, the time required to run a particular job is in part dependent upon the time required for drying of liquid media such as inks, varnishes or water-based acrylic coatings applied to a substrate. The term "drying" is used broadly herein, and includes oxidative polymerization of inks and varnishes commonly employed in sheet-fed lithographic presses, in addition to evaporation of water from water-based inks and coatings.
In some printing operations, such drying begins shortly after ink is applied with formation of a skin on the ink, i.e., "setting" of the ink, and continues after the printed stock is placed in a pile at a delivery station. In such operations, the temperature and heat capacity of the stock affect the rate of drying, and the core temperature of the accumulation of stock at the delivery station, i.e., "pile temperature", is ideally maintained at an elevated temperature, e.g., about 100.degree. F., until the process is substantially completed.
In a sheet-fed lithograph press, when a sheet is placed in a pile and later-printed sheets are continuously added to the pile, the weight of the later-printed sheets can cause undesirable transfer of ink, or "set-off", from the particular sheet to the next adjacent sheet, if the rate at which the ink dries on the particular sheet is not high enough relative to the rate at which subsequent sheets are added. Moreover, even after ink has sufficiently dried to avoid set-off in the pile, several hours of additional drying time may be required prior to subsequent finishing operations, additional printing, or delivery of the job, in order to avoid smearing or other deleterious disturbance of the printing.
In the case of water-based acrylic coatings, water is evaporated from the coating in a relatively short time period by infrared heating or blowing of hot air over the coating. It has been found that the rate of drying of such coatings is greatly affected by the rate of air flow. Pile temperature is generally not considered important, because the coating is substantially dried before the stock is stacked at the delivery station, and the coating prevents set-off whether or not the underlying ink is dry. Such coatings generally are applied only in high-pile presses. Low-pile presses generally are not capable of applying such coatings.
It is well known that employment of infrared heating units can reduce the need for spray powder and reduce turnaround time in the context of non-coated, printed stock, by increasing rates of drying inks. However, commercially available infrared dryers have several shortcomings. Infrared dryers are generally inefficient in drying water-based coatings, and typically employ radiators which may operate at temperatures above the combustion temperature of paper stock, thereby presenting potential fire hazards. Also, the high operating temperatures of infrared heating devices may result in undesirable and inefficient increases in ambient air temperature, and excessive heating of the press itself, accelerating wear. The heat generated during normal operation of infrared radiators may require water-cooling or exhaust systems to protect the press from excessive heat, and the energy consumption of such infrared radiators is generally undesirably high. Furthermore, the capital costs and operating expenses associated with such apparatus may render it economically unfeasible in certain contexts, particularly in low-pile presses.
There is a need for improved means to dry inks and coatings in printing presses, particularly in low-pile presses, in which dimensional constraints limit access to printed stock.